The smart folder also looks at when each message was received by the app, not just the date/time shown.
This is intentional - in case you're traveling some place where there is no connectivity, and then come back to civilization and the app catches up on old messages.
It also marks messages for the smart folder when an existing older message is found to have changed its read state (e.g. you've marked something unread in web mail, perhaps it's important?)
Maybe I should remove this logic as too confusing, and just use the cutoff value exactly as entered, based on message date/time stamps alone.
In any case, it should start working the way you expect as more time passes (e.g. if the setting is "one day", that would be one day from when you initially created the account... and from that point on...)
